You absolutely need a solid grounding in multi-variable calculus, linear algebra, probability theory and information theory. It will also be helpful to be well versed in graph theory. In my opinion one of the best starting points is "Information Theory, Inference and Learning Algorithms" by David MacKaye. It's a bit long in the tooth now, but it is still one of the most approachable and well written books in the fiel…
Free PDFs of some of the books mentioned: "Information Theory, Inference and Learning Algorithms" by David MacKaye http://www.inference.org.uk/itprnn/book.pdf "Probability Theory: the Logic of Science" by E. T. Jaynes http://www.med.mcgill.ca/epidemiology/hanley/bios601/Gaussia... "Elements of Statistical Learning" by Tibshirani https://web.stanford.edu/~hastie/Papers/ESLII.pdf "Bayesian Data Analysis" by Andrew Gelm…
edit: Goodfellow/Bengio/Courville, not mentioned in the previous comment, is also available online: http://www.deeplearningbook.org